drm/radeon: fix up pll selection on DCE5/6

Selecting ATOM_PPLL_INVALID should be equivalent as the
DCPLL or PPLL0 are already programmed for the DISPCLK, but
the preferred method is to always specify the PLL selected.
SetPixelClock will check the parameters and skip the
programming if the PLL is already set up.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>